Differentially geared reversible fan for ducted fan gas turbine engines

Abstract

A ducted fan gas turbine engine comprises a pair of turbines in flow series which are contra-rotating and which drive shafts which in turn drive into an epicyclic gear. The output from the epicyclic gear drives a fan which normally provides forward thrust from the engine. The fan can be made to rotate in the opposite direction to provide reverse thrust by varying the relative speeds of rotation of the two turbines by bleeding working fluid from between them.

We claim: 
     
       1. A ducted fan gas turbine engine comprising a fan, two shafts, a turbine drivingly connected to each shaft for driving the shafts in opposite rotational directions, and a gear mechanism, wherein the two shafts are connected with the gear mechanism, the output of the gear mechanism being connected to drive the fan, the gears of the gear mechanism being arranged so that a given change of the relative speeds of rotation of the two shafts causes a reversal of the direction of rotation of said output, and means for varying the relative speeds of rotation of the shafts for achieving said given change of relative speeds of rotation thereof. 
     
     
       2. A ducted fan gas turbine engine as claimed in claim 1 and in which the means for varying the relative speeds of rotation of the shafts comprises means for bleeding working fluid from between the two turbines to vary the pressure drop simultaneously across each turbine. 
     
     
       3. A ducted fan gas turbine engine as claimed in claim 1 and in which the gear mechanism is an epicyclic gear, comprising a sun gear, an annulus gear and a plurality of planet gears supported in a cage, the contra-rotating shafts being provided with gears which constitute the sun and annulus gears of the epicyclic gear, the cage connected for transmitting drive to the fan. 
     
     
       4. A ducted fan gas turbine engine as claimed in claim 1 and which includes low pressure and high pressure compressors and turbines and wherein the two shafts are concentric and contra-rotating, one of the two shafts carrying the low pressure compressor and the low pressure turbine, the other of the two shafts carrying the high pressure compressor and turbine. 
     
     
       5. A ducted fan gas turbine engine as claimed in claim 3 and in which the teeth of the annulus gear are formed on an internal surface of the radially outer shaft and the teeth of the sun gear are formed on an external surface of the radially inner shaft. 
     
     
       6. A ducted fan gas turbine engine as claimed in claim 1 and which includes a high pressure compressor drivingly connected by a shaft to a high pressure turbine, a low pressure compressor drivingly connected by a shaft to a low pressure turbine and a free turbine downstream of the low pressure turbine, wherein the shaft connecting the low pressure turbine and compressor constitutes a first one of the two shafts connected with the gear mechanism, and the free turbine is connected to the gear mechanism by a shaft which constitutes the second of the two shafts, said second of the two shafts extending concentrically through the first of the two shafts and contra-rotating in relation thereto. 
     
     
       7. A ductd fan gas turbine engine as claimed in claim 8 and in which the teeth of the annulus gear are formed on an internal surface of the second of the two shafts, and the teeth of the sun gear are formed on an external surface of an extension of the first of the two shafts. 
     
     
       8. A ducted fan gas turbine engine as claimed in claim 6 and in which the gear mechanism is an epicyclic gear, comprising a sun gear, an annulus gear and a plurality of planet gears supported in a cage, the contra-rotating shafts being provided with gears which constitute the sun and annulus gears of the epicyclic gear, the cage connected for transmitting drive to the fan.